radiation which arises from interactions of the primary radiation
beam with the atoms in the object being imaged. Because the
scattered radiation deviates from the straight line path between the
X-ray focus and the image receptor, scattered radiation is a major
source of image degradation in both X-ray and nuclear medicine
imaging techniques. When X-ray radiation passes through a patient,
three types of interactions can occur, including coherent scattering
(coherent scatter), photoelectric absorption and Compton scattering
. Of these three events, the great majority of scattered X-rays in
diagnostic X-ray imaging arise from Compton scattering.
In coherent scattering, the energy of the primary X-ray photon is
first completely absorbed and then re-emitted by the electrons of a
single atom. Because no net energy is absorbed by the atom, the
re-emitted X-ray has the same energy as the original X-ray, however
the direction of re-emission is totally arbitrary.
In photoelectric absorption, the energy of the X-ray photon is
completely absorbed as it ejects a tightly bound electron from one
of the atom's inner shells. The excess energy of the photon over
that of the binding energy of the electron is carried off as kinetic
energy by the ejected electron. Low energy characteristic radiation
is generated as an electron from an outer shell falls into the
vacated lower shell.
Finally, in Compton scattering, the interaction can be considered
as a collision between a high energy X-ray photon and one of the
outer shell electrons of an atom. This outer shell electron is bound
with very little energy to the atom and essentially all of the
energy lost by the X-ray photon in the collision is transferred as
kinetic energy to the electron, and the electron is ejected from the
atom. Because energy and momentum are both conserved in this
collision, the energy and direction of the scattered X-ray photon
depend on the energy transferred to the electron. When the initial
X-ray energy is high, the relative amount of energy lost is small,
and the scattering angle is small relative to the initial direction.
When the initial X-ray energy is small, the scattering is more
isotropic in all directions. At X-ray energies on the order of 1 MeV
(the energy range used in radiation therapy), the scattering is
mostly in the forward direction. At X-ray energies of 100 keV (the
diagnostic imaging range), the scattering is more isotropic.
The relative probability of the three types of interactions for
different materials is shown in Fig. 2. It is seen that in the
diagnostic imaging range, near 100 keV, Compton scattering comprises
the great majority of interactions for normal tissues in the body.
The probability of photoelectric interactions increases as the
substance atomic number increases (going from water to bone for
example) and as the X-ray energy decreases. Coherent scattering is
seen to be a very small fraction of the total number of scattering
Scattered X-rays that arise from Compton scattering constitute a
serious problem in diagnostic imaging. Although the scattered X-ray
photons are nearly isotropic in direction at diagnostic energies,
the scattered X-ray detected in the image are primarily forward
directed and thus have energies and angles of incidence near those
of the primary X-rays. Thus, these scattered X-rays cannot be
completely removed by the use of antiscatter grids or energy
filters. The residual scatter reduces radiographic contrast in X-ray
imaging and contributes to image intensity distortion in computed
tomography CT .